/* * Interface to code for recognizing formula categories in Horn * clause logic */ sig hc_syntax. accum_sig logic_types, logic_basic. /* an `input' predicate---only its definition is used here */ useonly atom form -> o. /* this predicate is used and its definition may be changed */ type termp term -> o. /* `output' predicates---this module supplies their complete definition */ exportdef goal form -> o. exportdef def_clause form -> o.